1. Field of the Invention
The present invention generally relates to a color converting device and, more particularly, to a device for converting a video signal, including R (red), G (green) and B (blue) signals into printing data including Y (yellow), M (magenta) and C (cyan) components or Y, M, C and K (black) components.
2. Description of the Prior Art
Examples of prior art color conversion method and apparatuses therefor are disclosed in, for example, the Japanese laid-open patent publications No. 58-178355 published Oct. 19, 1983; No. 60-220660 published Nov. 5, 1985; and No. 59-123392 published July 17, 1984.
According to the color conversion method disclosed in the first mentioned publication, the following simple matrix calculation is performed to give Y, M and C printing data. ##EQU1## wherein: ##EQU2## represents the Y, M and C printing data, ##EQU3## represents data of R, G and B signals for each picture element, and ##EQU4## represent a color conversion coefficient matrix.
The color conversion method disclosed in the first mentioned publication has a problem in that, since it is not possible to obtain a single color conversion coefficient utilizable to effect an adjustment appropriate to the spectral distribution and the ink transfer characteristic of pigments used in a printing ink, a relatively large error is involved in the color conversion.
The color conversion method disclosed in the second mentioned publication is aimed at substantially eliminating the problem inherent in the color conversion method discussed in the first mentioned publication. This color conversion method makes use of a plurality of sets of color conversion coefficient matrixes designed to give an optimum color conversion for each of a plurality of color space regions, so that a color conversion of favorable color reproducibility can be accomplished by selecting one of the color conversion coefficient matrixes which corresponds to the color space regions to which the R, G and B signals belong and then by carrying out a matrix calculation. Even with this color conversion method, the error in color conversion tends to become large at the boundary at which the color conversion coefficient matrixes are selected.
The color converting device disclosed in the last mentioned publication makes use of a look-up table wherein, by assigning an address to all the combinations of signal intensities r, g and b of the R, G and B image signals, Y, M, C and K color conversion data are stored in correspondence with a combination of the signal intensities r, g and b at such respective addresses. Although this color converting device is effective to give an optimum color reproducibility, there is, however, a problem in that the read-only memory used for the look-up table must have a relatively large capacity.
The color conversion methods and the devices therefor which are disclosed in the second and last mentioned publications will be discussed with the aid of some of the accompanying drawings.
FIG. 29 illustrates a circuit block diagram of the prior art color converting device disclosed in the second mentioned publication, that is, the Japanese laid-open patent publication No. 60-220660. The color converting device shown therein comprises a matrix multiplier 101, a color conversion coefficient matrix table 102 including a plurality of color conversion coefficient matrixes and a selector 103 for selecting one of the color conversion coefficient matrixes.
Assuming that the R, G and B signals are applied to the color conversion coefficient matrix selector 103, the selector 103 identifies, for each picture element, to which a plurality of regions, defined by dividing a color signal space having three axes represented by the respective intensities of the R, G and B signals, the R, G and B signals so applied belong, and then applies an identifying signal to the color conversion coefficient matrix table 102. The color conversion coefficient matrix table 102 is provided with a plurality of color conversion coefficient matrixes corresponding respectively to the regions in the color signal space and, therefore, upon receipt of the identifying signal from the selector 103, supplies one of the color conversion coefficient matrixes which corresponds to the identifying signal to the matrix multiplier 101. The matrix multiplier 101 performs a matrix calculation for each picture element, using the R, G and B signals and the color conversion coefficient matrix that has been selected by the color conversion coefficient matrix selector 103, thereby to provide the Y, M and C printing data.
Since each color conversion coefficient matrix is associated with a limited region within the color signal space and the color conversion coefficient is so selected that the average difference in color between the original image and the printed image can be minimum, the printing data capable of giving a favorable reproducibility can be obtained.
However, even the color conversion method discussed above has a problem in that the error in color conversion tends to be large at the switching point at which the selector 103 selects one of the color conversion coefficient matrixes. Therefore, unless one color conversion coefficient matrix is provided for each of the color attributes, that is, hue, saturation and lightness, represented by each of the R, G and B color signal components of the input video signal, no optimum color conversion can be attained.
A circuit block diagram of the color converting device disclosed in the last mentioned publication, that is, the Japanese laid-open patent publication No. 59-123392 is reproduced in FIG. 30 of the accompanying drawings. This prior art color converting device comprises a read-only memories (ROMs) 100 having respective address terminals to which the R, G and B signals are applied so that the color conversion can be accomplished by way of a table conversion of Y, M and C printing data stored at address locations in the read-only memories.
Generally, it is well known that the R, G and B signals to be converted into the printing data requires a data of 6 bits or more for each picture element. Assuming that this requires 6 bits, the number of addresses for a single color will be 2.sup.18 and one byte (8 bits) will be required for each address for each of the yellow, magenta and cyan colors and, therefore, ROM 100 must have a total capacity of about 6 megabits (.apprxeq.2.sup.18 .times.3.times.8).
Although the color converting device according to the last mentioned publication is also effective to accomplish an acceptable color conversion as is the case with that described with reference to FIG. 29, not only is the use of the high capacity read-only memory required, but also the preparation of the memory tables requires complicated procedures.
As hereinbefore discussed, according to the prior art color conversion methods and the devices therefor have problems in that a single color conversion coefficient matrix capable of satisfying printing requirements such as the three attributes of color (luminosity, hue and saturation) and the ink transfer characteristic having a non-linear property cannot be obtained, and in that, even when the plurality of the color conversion coefficient matrixes are used, the error in color conversion tends to be large at the boundary of the regions, to which the color conversion coefficient matrixes are allocated, to such an extent that a high fidelity image reproduction can no longer be attained when printed. Moreover, in the color converting device utilizing the table conversion, the read-only memory of relatively large capacity is required, posing a problem associated with the economy.
Accordingly, the present invention has been devised with a view to substantially eliminating the above discussed problems inherent in the prior art color conversion methods and the devices therefor and is aimed at providing an improved color converting device which can give a highly favorable color reproducibility which is simple in construction requiring a memory of relatively small capacity.